This invention relates to acoustic waveguides in general and more particularly to an improved type of waveguide which permits greater packing densities and other advantages.
Surface wave acoustic devices are gaining widespread use as filters, delay lines and the like. In particular, in frequency ranges between 10 mhz and 1 ghz, devices which are compact and provide numerous advantages over inductive-capacitive type filters and tuned electromagnetic waveguides are possible. This results directly from the fact that acoustic waves travel at a much slower speed than electromagnetic waves and thus, the size of a structure can be correspondingly smaller in the order of 10.sup.5.
When used in filtering applications these devices generally comprise a piezoelectric substrate on which are deposited two spaced transducers. The most common type of transducer used is what is known as the interdigital transducer wherein a plurality of fingers extend from the transducer pads on each side of the substrate and have overlapping portions. Electric fields created between the overlapping fingers of the transducer excite the piezoelectric material to generate the surface waves. Also used are what are known as grating mode transducers in which a grating of fingers coacts with a ground plane in much the same manner.
These conventional type waveguides where the waveguide is in the form of a plate, have waves therein which are referred to as Rayleigh modes. These conventional surface waves are subject to loss of energy due to diffraction which necessitates the use of acoustic beams that are many wave lengths wide. Such an arrangement offers certain disadvantages, particularly when it is desired to densely pack a plurality of waveguides.
To overcome some of these problems, waveguides which propagate in the lowest order anti-symmetric flexural mode, have been developed. Generally, the proposed waveguides have been vertically oriented structures such as that illustrated by FIG. 1. Considerable theoretical analysis has been performed on the performance of such a waveguide. Its dispersion as a function of guide cross sectional geometry has been computed as described in an article by P. E. Lagasse, I. M. Mason and E. A. Ash, entitled "Acoustic-Surface Waveguides--Analysis and Assessment," IEEE Trans MTT 21 No. 4, 225-235 (April, 1973), and in another article by R. Burridge and E. J. Sabina entitled "The Propagation of Elastic-Surface Waves Guided by Ridges," Proc. R. Soc. Lond. A. 330 pps. 417-441 (1972). The deformation associated with the different modes that the guide supports have been studied in the Burridge et al. article above and also in an article by C. C. Tu and G. W. Farnell entitled "Flexural Mode of Ridge Guides for Elastic Surface Waves," Elec Lett. 8 No. 3 pps. 68-69, (Feb. 10, 1972). In the last mentioned article, the field pattern penetration into the substrate was also presented.
In waveguides of this nature, the most tightly confined mode has a ridge structure as illustrated by FIG. 2A. The computer generated flexure illustrated by that figure shows that a substrate is virtually undisturbed by the presence of the mode in the guide. It is the property of the guide that makes it especially important since it leads to the possibility of a high density of non-interacting acoustic channels adjacent to each other on a common substrate. The graph of FIG. 2C illustrates the typical dispersion of this mode, i.e., the lowest order anti-symmetrical flexural mode is illustrated by the curve 11. The fact that the velocity of this mode can be very low is desirable in achieving long time delays. However, the dispersion evident in the lower branch is not desirable unless controllable.
The other category of mode guided by the ridge is the modified form of the Rayleigh wave. When a ridge is present on a half space, the Rayleigh mode becomes slowed down by the ridge and its amplitude falls off away from the ridge. This is illustrated by the computer generated flexural pattern of FIG. 2B. The inability of the guide to confine the energy of this mode makes it relatively uninteresting, particularly when it is noted that the guide has added dispersion for this mode. Typical dispersion for this mode is illustrated by the upper curve 13 of FIG. 2C.
Prior experimental efforts in making waveguides of this nature have taken basically two approaches. In one approach, thin vertically oriented rectangular ridges, such as that of FIG. 1, of aluminum have been machined. By its very nature, this approach did not permit fine geometries to be achieved and consequently the guides were limited to low frequency operation, i.e., less than 5 mhz. In addition, the nature of this type of approach results in a structure in which a high density of adjacent guides cannot be fabricated. Furthermore, the excitation of the guides presents severe problems. Since aluminum is not a piezoelectric material, a separate transduction means must be attached to the guide for the excitation of the modes. In the second approach which is disclosed in a paper by I. M. Mason, M. D. Motz and J. Chambers entitled "Wedge Waveguide Parametric Signal Processing," Proc. Ultrasonics Symposium, Boston, pps. 314-315, (October, 1972), one side of a piezoelectric substrate is lapped to produce a wedge shaped structure. The tip of the wedge then acts as the guiding ridge. This technique permits easy excitation of the mode since the substrate is piezoelectric. However, dispersion is difficult to control because of the craftsman-type construction method and a high density of guides is impossible to achieve. The main disadvantages of the prior art approaches are as follows:
1. Use of vertically oriented ridges does not lead to highly repeatable geometries and a high density of waveguides at VHF frequencies, unless substrates are used that admit to orientation dependent etching so that guide walls can be defined precisely by crystal planes. Presently, the material capability in this regard exists only for silicon which is a non-piezoelectric material. In turn, the use of silicon may lead to significant technological difficulties in the excitation of the required modes discussed below;
2. Even if highly precise repeatable guide geometries could be produced, the dispersion of the guide is not amenable to alteration, i.e., there is no parameter available for modification that will permit the dispersion in certain range of wave lengths to be flattened;
3. The motion of the guide is essentially parallel to the plane of the substrate and thus, not along the deformation direction of currently sputtered piezoelectric material.
Thus, it can be seen that although waveguides excited by tightly confined modes can offer distinct advantages, there is a need for an improved waveguide of this nature which does not suffer the above-noted drawbacks.